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<div class="titlepage"><div><div><h3 class="title">
<a name="fiber.callbacks.return_errorcode_or_data"></a><a class="link" href="return_errorcode_or_data.html" title="Return Errorcode or Data">Return Errorcode
      or Data</a>
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<p>
        Things get a bit more interesting when the async operation&#8217;s callback passes
        multiple data items of interest. One approach would be to use <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;&gt;</span></code> to capture both:
      </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</span> <span class="identifier">read_ec</span><span class="special">(</span> <span class="identifier">AsyncAPI</span> <span class="special">&amp;</span> <span class="identifier">api</span><span class="special">)</span> <span class="special">{</span>
    <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</span> <span class="identifier">result_pair</span><span class="special">;</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">promise</span><span class="special">&lt;</span> <span class="identifier">result_pair</span> <span class="special">&gt;</span> <span class="identifier">promise</span><span class="special">;</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">future</span><span class="special">&lt;</span> <span class="identifier">result_pair</span> <span class="special">&gt;</span> <span class="identifier">future</span><span class="special">(</span> <span class="identifier">promise</span><span class="special">.</span><span class="identifier">get_future</span><span class="special">()</span> <span class="special">);</span>
    <span class="comment">// We promise that both 'promise' and 'future' will survive until our</span>
    <span class="comment">// lambda has been called.</span>
<span class="preprocessor">#if</span> <span class="special">!</span> <span class="identifier">defined</span><span class="special">(</span><span class="identifier">BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES</span><span class="special">)</span>
    <span class="identifier">api</span><span class="special">.</span><span class="identifier">init_read</span><span class="special">([</span><span class="identifier">promise</span><span class="special">=</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span> <span class="identifier">promise</span><span class="special">)](</span> <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">data</span><span class="special">)</span> <span class="keyword">mutable</span> <span class="special">{</span>
                            <span class="identifier">promise</span><span class="special">.</span><span class="identifier">set_value</span><span class="special">(</span> <span class="identifier">result_pair</span><span class="special">(</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">data</span><span class="special">)</span> <span class="special">);</span>
                  <span class="special">});</span>
<span class="preprocessor">#else</span> <span class="comment">// defined(BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES)</span>
    <span class="identifier">api</span><span class="special">.</span><span class="identifier">init_read</span><span class="special">(</span>
            <span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span><span class="special">([](</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">promise</span><span class="special">&lt;</span> <span class="identifier">result_pair</span> <span class="special">&gt;</span> <span class="special">&amp;</span> <span class="identifier">promise</span><span class="special">,</span>
                          <span class="identifier">AsyncAPI</span><span class="special">::</span><span class="identifier">errorcode</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">data</span><span class="special">)</span> <span class="keyword">mutable</span> <span class="special">{</span>
                            <span class="identifier">promise</span><span class="special">.</span><span class="identifier">set_value</span><span class="special">(</span> <span class="identifier">result_pair</span><span class="special">(</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">data</span><span class="special">)</span> <span class="special">);</span>
                  <span class="special">},</span>
                  <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span> <span class="identifier">promise</span><span class="special">),</span>
                  <span class="identifier">std</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">::</span><span class="identifier">_1</span><span class="special">,</span>
                  <span class="identifier">std</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">::</span><span class="identifier">_2</span><span class="special">)</span> <span class="special">);</span>
<span class="preprocessor">#endif</span> <span class="comment">// BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES</span>
    <span class="keyword">return</span> <span class="identifier">future</span><span class="special">.</span><span class="identifier">get</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
      </p>
<p>
        Once you bundle the interesting data in <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;&gt;</span></code>, the code is effectively identical
        to <code class="computeroutput"><span class="identifier">write_ec</span><span class="special">()</span></code>.
        You can call it like this:
      </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">data</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">read_ec</span><span class="special">(</span> <span class="identifier">api</span><span class="special">);</span>
</pre>
<p>
      </p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2013 Oliver Kowalke<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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